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Contract Law
Part of the common law series
Contract theory
Contract formation
Offer and acceptance  · Mailbox rule
Mirror image rule  · Invitation to treat
Consideration
Defenses against formation
Lack of capacity to contract
Duress  · Undue influence
Illusory promise  · Statute of frauds
Non est factum
Contract interpretation
Parol evidence rule
Contract of adhesion
Integration clause
Contra proferentem
Excuses for non-performance
Mistake  · Misrepresentation
Frustration of purpose  · Impossibility
Unclean hands  · Unconscionability
Illegality  · Accord and satisfaction
Rights of third parties
Privity of contract
Assignment  · Delegation
Novation  · Third party beneficiary
Breach of contract
Anticipatory repudiation  · Cover
Exclusion clause  · Efficient breach
Fundamental breach
Remedies
Specific performance
Liquidated damages
Penal damages  · Rescission
Quasi-contractual obligations
Promissory estoppel
Quantum meruit
Subsets: Conflict of law Commercial law
Other areas of the common law
Tort law  · Property law
Wills and trusts
Criminal law  · Evidence

The parol evidence rule enacts a principle of the common law of contracts that presumes that a written contract embodies the complete agreement between the parties involved. The rule therefore generally forbids the introduction of extrinsic evidence (i.e., evidence of communications between the parties which is not contained in the language of the contract itself) which would change the terms of a later written contract.

In order for the rule to be effective, the contract in question must be an integrated writing; it must, in the judgment of the court, be the final agreement between the parties (as opposed to a mere draft, for example). One way to ensure that the contract will be found an integration is through the inclusion of a merger clause, which recites that the contract is, in fact, the whole agreement between the parties. However, many modern cases have found merger clauses to be only a rebuttable presumption.

An integrated agreement is either a partial or complete integration. If it contains some, but not all, of the terms as to which the parties have agreed then it is a partial integration. This means that the writing was a final agreement between the parties (and not mere preliminary negotiations) as to some terms, but not as to others. On the other hand, if the writing were to contain all of the terms as to which the parties agreed, then it would be a complete integration. The importance of this distinction is relevant to what evidence is excluded under the parol evidence rule. For both complete and partial integrations, any evidence contradicting the writing is excluded under the parol evidence rule. However, for a partial integration, terms that do not contradict the writing but merely add to it are not excluded.

There are a number of exceptions to the parol evidence rule. Extrinsic evidence can always be admitted for the following purposes:

• To work out the subject matter of the contract.
• To resolve an ambiguity in the contract.^[1]
• To show that an unambiguous term in the contract is in fact a mistaken transcription of a prior valid agreement. Such a claim must be established by clear and convincing evidence, and not merely by the preponderance of the evidence.
• To show fraud, duress, mistake, or illegal purpose on the part of one or both parties.
• To show that consideration has not actually been paid. For example, if the contract states that A has paid B $1,000 in exchange for a painting, B can introduce evidence that A had never actually conveyed the $1,000.
• To identify the parties, especially if the parties have changed names.
• To imply or incorporate a term of the contract.

In order for evidence to fall within this rule, it must involve either (1) a written or oral communication made prior to execution of the written contract; or (2) an oral communication made contemporaneous with execution of the written contract. Evidence of a later communication will not be barred by this rule, as it is admissible to show a later modification of the contract (although it might be inadmissible for some other reason, such as the Statute of Frauds. Similarly, evidence of a collateral agreement - one that would naturally and normally be included in a separate writing - will not be barred. For example, if A contracts with B to paint B's house for $1,000, B can introduce extrinsic evidence to show that A also contracted to paint B's storage shed for $100. The agreement to paint the shed would logically be in a separate document from the agreement to paint the house.

Though its name suggests that it is a procedural evidence rule, the consensus of courts and commentators is that the parol evidence rule constitutes substantive contract law.

Additional information on the parol evidence rule may be found in Restatement 2d of Contracts § 213.

1. ^ Pacific Gas & Elec. Co. v. G. W. Thomas Drayage Co., 69 Cal. 2d 33 (1968). Pacific Gas & Electric is one of Roger Traynor's most famous (and controversial) opinions.